A device that senses if an illuminated room is empty, and turns off the light.

Waste not, want not - a saying made for the lunar environment.

As they left their joint office, Strong, always penny conscious, was careful to switch off the light. Harriman had seen him do so a thousand times; this time he commented. "George, how about a light switch that turns off automatically when you leave a room?"

"Hmm- but suppose someone were left in the room?"

"Well... hitch it to stay on only when someone was in the room- key the switch to the human body's heat radiation, maybe."

"Too expensive and too complicated."

"Needn't be. I'll turn it over to Ferguson to fiddle with. It should be no larger than the present light switch and cheap enough so the power saved in a year will pay for it..."